select rows with non distinct values in column 1 scoped to column 2

Question

So i have a people table and bank_accounts table:

People
id | name
1    John
2    Mark
3    Mary

BankAccount
id | person_id | currency
1      1          'USD'
2      1          'EUR'
3      2          'USD'
4      2          'USD'
5      3          'EUR'

I want to get back all accounts with their owners if that owner has only accounts with max one kind of currency. I don't want to get back any account which is owned by user which has another account in another currency. erm :P

so the table i want to get back looks like that:

account_id | person_id | currency

3            2(Mark)     'USD'
4            2(Mark)     'USD'
5            3(Mary)     'EUR'

Hope it's understandable. This is simplified example of course. I will use this on a lot bigger tables with a lot of data. So some efficiency would be also good.

Thanks a lot for your time and help!

Answer 1

select * 
from bankAccount
where person_id in
(
  select person_id
  from bankAccount
  group by person_id
  having count(distinct currency) = 1
)